The present invention relates to an electrical charging device of a magnetic brush type for charging an image-forming object with electricity uniformly in an image forming apparatus such as an electrophotographic copying machine and a printer of an electrophotographic type.
In an image forming apparatus of an electrophotographic type, a corona electrical charging unit has generally been used for charging an image forming object such as a photoreceptor drum. In the corona electrical charging unit, high voltage is impressed on a discharge wire to generate a strong electric field around the discharge wire for gas discharge, and an image forming object ads orbs charged ions produced in the gas discharge to be charged with electricity.
The aforementioned corona electrical charging unit used in conventional image forming apparatuses of an electrophotographic type has an advantageous point that an image forming object is not damaged in the course of charging, because the image forming object does-not come into contact with the corona electrical charging unit during charging. However, the corona electrical charging unit has disadvantageous points that an electric shock and a leak are feared due to high voltage used and ozone generated in gas discharge is harmful for human bodies and it shortens life of the image forming object. Further, charging voltage by means of the corona electrical charging unit is unstable because it is affected greatly by temperature and humidity. In addition to the above, the corona electrical charging unit generates a noise that is caused by high voltage, and 5 seconds or more are needed after the input of high voltage to obtain stable charging voltage, which has been the greatest drawback for utilizing an image forming apparatus of an electrophotographic type as a communication terminal or an information processing equipment.
The cause of such many drawbacks of the corona electrical charging unit lies in the fact that charging is performed in gas discharge.
As an electrical charging device which does not perform gas discharge by means of high voltage as in a corona electrical charging unit, does not harm mechanically to an image forming object, and can charge electrically the image forming object, Japanese Patent Publication Open to Public Inspection No. 133569/1984 (hereinafter referred to as Japanese Patent O.P.I Publication) discloses an electrical charging device wherein magnetic particles are adsorbed on a cylindrical conveying carrier containing therein magnetic objects to become a magnetic brush which rubs the surface of an image forming object under impression of D.C. bias voltage so that charging may be performed.
The magnetic brush mentioned above can charge an image forming object electrically without damaging it because the magnetic brush is a flexible brush composed of magnetic particles, and it is more excellent than other charging units of a contact type such as a fur brush charging unit and a charging unit employing a conductive and elastic roll. However, even in the case of using a charging unit employing the magnetic brush mentioned above, uniform charging can not always be obtained and there have occurred some problems that magnetic particles are stuck to the image forming object.
With the background mentioned above, Japanese Patent O.P.I. Publication Nos. 21873/1992 and 116674/1992, for example, disclosed a magnetic brush type charging method wherein A.C. bias voltage containing D.C. components is impressed on a magnetic brush for charging an image forming object. According to the description in Japanese Patent O.P.I. Publication No. 21873/1992, the aforementioned A.C. bias voltage is established for charging so that its peak to peak voltage V.sub.p-p exceeds the threshold value for discharge of the magnetic brush, and preferable frequency of A.C. bias voltage is 100-500 Hz. Further, magnetic particles used are to be iron, iron oxide or ferrite particles whose particle size is 20-200 .mu.m and specific resistance is 10.sup.5 -10.sup.9 .OMEGA..multidot.cm, thereby uniform charging can be provided on the image forming object. According to the description in Japanese Patent O.P.I. Publication No. 116674/1992, it is preferable that a magnetic particle having a particle size of not more than 70 .mu.m and not less than 30 .mu.m, a specific resistance of not less than 10.sup.5 .OMEGA..multidot.cm and not more than 10.sup.12 .OMEGA..multidot.cm and a shape which is spherical to the utmost is used and a distance between a conveying carrier and an image carrier is in the range of 100-5000 .mu.m for charging.
However, charging methods described in the aforementioned publications still have problems that uneven charging in the pattern of streaks or ring marks is caused on the surface of the image forming object due to a coarse bristle formed with magnetic particles, or magnetic particles are stuck to the image forming object. When temperature and humidity are low, in particular, magnetic particles tend to stick because their resistance becomes high.